The present invention relates to a mixing apparatus.
The operation of mixing is generally understood to comprise two distinct actions; dispersive mixing and distributive mixing. In dispersive mixing the individual parts of the materials being mixed, whether solid or fluid, have their respective geometries altered by means of applied stresses. This usually takes the form of reducing the average size of individual parts while increasing their numbers. In distributive mixing the individual parts of the materials, whether solid or fluid, are blended together in order to obtain a spatial uniformity in the distribution of the various material parts with respect to one another. A good mixing operation thus usually requires both dispersive and distributive mixing actions to occur.
Distributive mixing is primarily a function of the geometry of the mixing apparatus and known mixers typically fall into two general types providing either random or structured distributive mixing. Random distributive mixers achieve mixing by randomly agitating the materials and include known mixers such as tumble-blenders and ribbon-blenders. Structured-distributive mixers on the other hand achieve mixing by systematically repeating a geometrically controlled sequence of dividing, reorienting and rejoining the materials and include static mixers and cavity transfer mixers.
In contrast, dispersive mixing is primarily a function of forces, pressures, stresses and strains applied to the materials. In general, the size reduction of materials that is required in dispersive mixing is achieved by applying stresses to the materials. These applied stresses usually take the form of compressive, tensile or shear stresses. For mixing fluid materials the predominant method of stressing has been by means of applying shear, as this can readily be achieved by utilising the drag forces that exist within a fluid bounded by two relatively moving surfaces in a machine. Examples of such mixers include internal rotor/stator mixers in which the material is sheared between the rotor and the stator surfaces. Shear stressing can also be obtained by forcing a fluid material over one or more surfaces that do not have a motion relative to one another, for instance between the walls of a channel. In this case it is still possible to generate significant shear stresses in the fluid, but only at the expense of providing some form of pumping energy to propel the fluid over the surfaces. It has long been recognised however that an alternative mechanism, that of extensional flow, is capable of subjecting fluid materials to compressive and tensile stresses that in practice can be much higher than the shear stresses.
Extensional flow requires that the fluid be pressurised in order to propel it between surfaces that subject the fluid to tensile or compressive stresses. Such surfaces can be generally orientated in the direction of the flow in which case the flowing material is accelerated or decelerated along its flowpath by virtue of mass conservation, or generally orientated across the direction of the flow, in which case the flowing material is decelerated and thus compressed by virtue of the change in the momentum of the fluid, such as in impact. Known mixers designed to operate on the basis of extensional flows for dispersion have thus required external means of pressurisation in the form of high-pressure pumps located upstream (the same requirement for pumping applies to a mixer operating on the basis of shear flow between non-moving surfaces as mentioned above). Given that it is often a requirement that any given part of the material being mixed is subjected to a number of stressing cycles it is apparent that the overall pressures required to provide extensional flows and shear flows through a mixer can become prohibitively high. Additionally, the need to engineer such a mixer so as to ensure that the extensional flow and shear flow occur with maximum efficiency, i.e. the minimum pressure loss, is relatively costly.
It is an object of the present invention to provide a mixing apparatus which obviates or mitigates the above disadvantages.
According to the present invention there is provided a mixing apparatus for mixing a material, the apparatus comprising one or more flow channels and at least two members which are either eccentrically mounted one within the other so as to define a chamber therebetween, or axially mounted defining a chamber between facing surfaces thereof, and which are rotateable relative to one another to thereby produce a pumping force to force material through said flow channels and chamber to thereby subject the material to stresses within said flow channels and/or said chamber that result in extensional-dispersive and/or shear-dispersive mixing.
Preferably the apparatus is further adapted to subject the material to distributive mixing.
The mixer preferably comprises a plurality of said stress-inducing flow channels in at least two sets defined by respective channel members arranged such that material is pumped from channels of one set to channels of another.
Pumping force may be imparted to the material during and/or intermediate two sets of said stress-inducing flow channels.
The channels may have sides that are parallel, convergent or divergent relative to one another and any channel may be entirely contained within a single channel defining member of the mixer or alternatively may be formed within the surface of one channel member and bounded by the adjacent surface of any other component of the mixer (e.g. another channel defining member). The channels may be, for instance, radial channels within generally concentric members or axial channels within members juxtaposed in an axial direction.
Chambers are preferably provided between channel defining members of the mixer the chambers providing random-distributive and both shear-dispersive and extensional-dispersive mixing to the mixing components. The chambers may, for instance, be annular spaces between concentric or eccentric surfaces, or be axial spaces between surfaces that are parallel or not-parallel. The chambers may be sufficiently small so as to permit the channel members to come into contact.
The pumping actions may, for instance, arise from centrifugal forces or from drag forces, or may take the form of positive-displacement pumping such as vane pumping, gear pumping or piston pumping.
In preferred embodiments of the invention there is provided means to obtain an amount of backflow mixing, in which the direction of the flow within a channel (or chamber between sets of channels) is reversed during part of the pumping cycle as a result of a reversal in the direction of the pressure differential across the channel (or chamber). The amount of flow occurring in the reverse direction may be controlled by means of the design of the channel (or chamber), singly or in combination, in which flow in one direction is subjected to a greater resistance than it is in the opposite direction. In this instance, the channels (or chamber) can be designed to operate as valves that permit more flow in one direction than they do in another, while at the same time being capable of imparting the appropriate mixing actions to the materials. Alternatively, the amount of flow occurring in the reverse direction may be controlled by means of the design of the pumping actions in which a greater pumping effect is achieved in one direction than it is in the other. This backflow can have a beneficial effect in increasing the residence time within the mixing unit, thereby subjecting any part of the material to an increased number of mixing actions. In some embodiments of the invention there may be no net flow in any one direction during mixing so that the mixing operation is essentially static (the mixer could have a common inlet/outlet).
Apparatus in accordance with the present invention can be used to mix a single material (the term mixing in this context is used throughout the mixing industry referring to, for example, dispersive mixing of a material to break it down into smaller component parts which may be coupled with distributive mixing in distributing those smaller parts through the material as a whole) or a number of different materials including mixtures of fluids and solids, or indeed just solids which are capable of behaving in a manner analogous to fluids.